The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
As one of the core techniques of Third Generation (3G) mobile communication, intelligent antenna technique can be used to produce a spatially directed wave beam according to the difference in signal space characteristic between mobile subscribers, so as to align the main lobe of antenna to the direction of arrival of subscriber signals and align the side lobe to the direction of arrival of interference signals, and thereby attain the purpose of utilizing mobile subscriber signals efficiently and eliminating or suppressing interference signals, improve efficiency of radio spectrum utilization and signal transmission, and utilize limited channel resource as far as possible. Compared to non-directional antennas, directional antennas can increase antenna gain in uplink and downlink greatly, reduce transmitted power level, improve Signal-to-Noise Ratio (SNR), and effectively overcome channel fading. In addition, since the antenna points to the subscribers directly, the interference between subscribers in the cell and between subscribers in adjacent cells is reduced, and the multipath effect is reduced.
To produce a spatially directed wave beam with intelligent antenna, a feed network device (i.e., beam shaping network) is required. As shown in FIG. 1, the feed network device is a main component of the antenna feeder subsystem in the base station system in 3G mobile communication system; the antenna feeder subsystem is connected to a duplexer in the base station system, and includes a feed network device, a power divider, and an antenna array, which are connected in sequence. A signal beam emitted from the Transmitter (TX) in the base station system is shaped and then transmitted to an antenna array, and a feed is provided to the array antenna unit, so that the antennae produce a plurality of separate spatially directed beams, and thereby afford good orientation to the superimposed electromagnetic wave. By guiding a radio signal to a specified subscriber direction, the subscriber can transmit and receive a signal in a limited directional area, and therefore the communication coverage and system capacity can be increased greatly, the spectrum utilization can be improved, the emission power in the base station can be reduced, the system cost can be reduced, and the interference between signals and the pollution of the electromagnetic environment can be reduced. In addition, since the Receiver (RX) also employs a plurality of separate antennae, the receiving sensitivity in an expected direction can be enhanced, and the signals in an unexpected direction can be suppressed.
In the prior art, a Butler matrix structure is usually used to implement a feed network device; Butler matrix structure is a passive and interchangeable circuit, which includes several couplers and phase shifting components, wherein, the couplers are two-input and two-output passive devices.
In the prior art, a feed network device that provides equal-amplitude output is implemented with 3 dB branch line directional couplers in standard Butler matrix topology structure; the feed network device is mainly composed of four 3 dB branch line directional couplers and two 45° phase shifters cascaded on a Printed Circuit Board (PCB). A 3 dB branch line directional coupler is a coupler that provides equal-amplitude output, and a signal at the input port becomes two output signals with an amplitude equal to half of the amplitude of the input signal after passing through the 3 dB branch line directional coupler.
FIG. 2 shows the topological structure of a feed network device implemented with 3 dB branch line directional couplers, wherein, the output pin1 of the 3 dB branch line directional coupler 201 is connected to the input pin3 of the 3 dB branch line directional coupler 202 via the 45° phase shifter 205, the output pin2 of the 3 dB branch line directional coupler 201 is directly connected to the input pin4 of the 3 dB branch line directional coupler 203, the 3 dB directional coupler 204, the 45° phase shifter 206, and the other two 3 dB branch line directional couplers are connected in a similar way.
After a signal enters the input pin Input1 of the 3 dB branch line directional coupler 201, a part of the signal is output from pin1 at the coupling port into the 45° phase shifter 205, and then is output from the input pin3 of the 3 dB branch line directional coupler 202 into the 3 dB branch line directional coupler 202, and is output from the pins Output1 and Output3 after passing through the 3 dB branch line directional coupler 202, respectively; the other part of the signal passed through the 3 dB branch line directional coupler 201 is output from the direct connection pin2 of the 3 dB branch line directional coupler 201 into pin4 of the 3 dB branch line directional coupler 203 directly, and is output from pins Output2 and Output4 after passing through the 3 dB branch line directional coupler 203.
Since two stages of 3 dB branch line directional couplers are used, after the signal is output at equal amplitude from the first coupler stage, the signals entering into the second coupler stage are output at equal amplitude further. Therefore, the feed network device can be used to divide equally the signal power input from any input port into four outputs at the output port.
Typical feed network devices with equal-amplitude output are implemented with branch line directional couplers in the prior art, a main line and a branch line of a branch line directional coupler are arranged in a surface layer of the PCB respectively, with air as a dielectric at one side and PCB material as a dielectric at the other side; therefore, the dielectric constant at the main line side is different to the dielectric constant at the branch line side, which causes poor electrical performance of the feed network device.